As wireless devices are becoming increasingly faster and smaller, the power consumption, reliability, and performance of a voltage controlled oscillator (VCO) in such devices have become a design focus. A VCO is typically used in modulating and demodulating a radio frequency (RF) signal by mixing the un-modulated or modulated signal with a locally generated oscillating signal. For example, when demodulating a RF signal, it is important to accurately match the VCO's generated oscillating signal with the RF's carrier signal. This is especially critical in high frequency application, as phase noise and jitters are more prevalent and detrimental to the performance of the VCO.
In high frequency application, the accurate generation of a high frequency oscillating signal depends on several factors such as the tunability of the VCO, Q factor, and the parasitic elements of the VCO's components. The tunability factor of the VCO is determined by the change of frequency generated by the VCO over the change of the input control voltage. Even though a VCO could be optimally designed for a specific high frequency application, manufacturing process tolerances will invariably introduce inaccuracies into the VCO. Thus it is important for a VCO to be tunable. Generally, it is desirable for a VCO to have a wide range of tunable frequency. However, broad tuning capability increases the VCO's susceptibility to noise and system variations due to the enhanced tuning sensitivity. As a result, it is important to maintain a high Q factor by lowering the parasitic components of the VCO's circuit. VCO tuning is implemented using a LC tank circuit including one or more inductors and capacitors.
Phase noise of the VCO can be reduced by maintaining a high Q factor in the LC tank. A high Q in the LC tank not only reduces phase noise but also increases performance and reduces power consumption of the VCO. To maintain a high Q in the LC tank, the parasitic components of the VCO's circuit must be reduced.
There are three parasitic components in a semiconductor device. They are resistive, capacitive, and inductive. These parasitic components negatively affect the reliability, performance, and power consumption of the device. In high frequency application, the inductors and capacitors of the VCO are generally integrated into a semiconductor device. Several methods are available to reduce the parasitic components of the device as a whole. For example, in fabricating an inductor, trenches or void space are introduced into a substrate layer directly underneath the inductor. The same method can also be used in fabricating the capacitor.
Typically, reduction of parasitic components are focused on the inductors and the capacitors. But as a semiconductor device gets progressively smaller it becomes more sensitive to signal propagation delays caused by parasitic resistance and capacitance inherent to the interconnection lines within the device. A wider interconnection line could be used to ease this problem; however, this will increase the parasitic capacitance of the line to the ground. Even though the parasitic capacitance and resistance of an interconnection line may seem small by itself, collectively it could reduce the Q factor of the semiconductor device. Thus to increase the performance of the semiconductor device and to maintain a high Q factor, it is necessary to reduce the parasitic components inherent to the interconnection lines.
Accordingly, there is a need in the art for a semiconductor device and a method to fabricate such a semiconductor device with reduced parasitic resistance and capacitance.